The present invention relates to an automatic focus detecting device for use with television cameras and motion picture cameras having a zoom lens and more particularly, to an automatic focus detecting device in which intervals at which a time sequence signal of two images formed by light beams passed through two different areas in a zooming lens system of the zoom lens is extracted are variable for obtaining a correlation between the two images.
Generally, in conventional TTL (through-The-Lens) type automatic focus detecting devices according to the base length type distance metering system, double images of an object are respectively obtained from two restricted light beams passed through different areas in a zooming lens system of a taking lens and the amount of shift between the posiitons at which the respective images are formed is measured for determining the focusing position. The above described type of automatic focus detecting device is more fully disclosed, for example, in Japanese Pat. unexamined publication No. 101111/'81. The principle of the above described type of automatic focus detecting device will now be explained by referring to FIG. 1.
In the automatic focus detecting device shown in FIG. 1, restricted light beams from an object passed through different areas A and B in a focal lens system 1 comprising focusing lens system 1a and zooming lens system 1b are caused to enter a beam splitter 2 so as to be divided into two pairs of light beams; one for image forming and the other for focus detecting. The image forming light beams are directed by an image forming lens system 3 to an image pick-up surface 4 of an image pick-up tube, self-scanning type photoelectric arrays or the like to form an image of the object thereon. The light beam for focus detection passed through the area B is caused to enter, through an image forming lens 5, the right side of a self-scanning type photographic array or image sensor 7 which may take the form of a charge coupled device (CCD) image sensor having a light receiving section and a transfer section. The other light beam passed through the area A is also caused to enter, through an image forming lens 6, the left side of the CCD image sensor 7. Upon moving the focusing lens system 1a along the optical axis 13 thereof, the images on the CCD image sensor 7 are caused to move in the opposite directions because the areas A and B are on a straight line 14 intersecting the optical axis 13 and are equidistant from the optical axis 13. When the optical image of the object on the image pick-up surface 4 is just focused, the amount of shift between the two images on the CCD image sensor 7 is equal to a predetermined value. On the other hand, when the optical image of the object is out of focus, that is, the image is focused in either front or rear of the image pick-up surface, the amount of shift between the two images on 7 is increased or decreased from the predetermined value depending on how much the focusing lens system is shifted.
The CCD image sensor 7 is driven by a CCD drive circuit 8 so as to provide a time sequence video signal corresponding to picture elements of the images formed on the CCD image sensor 7. In FIG. 2(a) which is a diagram showing a time sequence video signal of the images formed on the CCD image sensor 7, the reference numerals 15 and 16 designate the time sequence video signals of the images formed by the image forming lenses 5 and 6, respectively. The time sequence video signals 15 and 16 are directed to a binary circuit 9 to be converted into bistable state signals at a high logic level (hereinafter referred to as "H", when applicable) and a low logic level (hereinafter referred to as "L", when applicable) as shown in FIG. 2(b). A conventionally well-known type of signal converter according to a differential method, comparison method or the like is employed as the binary circuit 9. The bistable signals of the time sequence video signal are directed to a correlator 10 such as, for example, TC1004J (trade name) made by TRW company which has two shift registers capable of separately storing two series of the digital video signals by images shown in FIGS. 2(c) and (d), respectively. For providing the two series of the digital video signals, 64 CCD pixels to each side of the optical axis of the image forming lens 5, and consequently 128 pixels in total are allocated to each image formed by the light beam passed through the area B, on the other hand, 128 pixels to each side of the optical axis of the image forming lens 6 and consequently 256 pixels in total are allocated to the other image formed by the light beam passed thorugh the area A. The digital video signal attributable to the light beam passed through the area B (hereinafter referred to as "B data") is utilized as a reference signal with which the digital video signal attributable to the light beam passed through the area A (hereinafter referred to as "A data") has to be compared. By comparing the A data with the B data upon shifting the former relative to the later, the correlator 10 can provide a correlation signal (shown in FIG. 2(e)) representing the degree of agreement between these video signals which in turn is directed to an operation circuit 11. Specifically, when utilizing two shift register each having 128 bits, the correlation signal showns a degree of agreement of 128 pairs of signals from the respectively corresponding pixels. The correlation signal, which is variable between, for example, 0 (zero) and 5 volts, has the maximum value of 5 V when complete agreement between the two video signals stored in the respective shift registers occurs. Furthermore, the correlation signal shows 5 V after shifting the two video signals by 64 bits relative to each other when the focusing lens system 1a is in a focusing position.
A peak value of the correlation signal shown in FIG. 2(e) is detected by a peak hold circuit included in an operation circuit 11 in such a way as is shown in FIGS. 2(f) and (g). The operation circuit 11 can caluculate and amount of shift of the peak value from a reference value (at the position of the 64th bit in the above example) and direct it to a drive circuit 12. For automatically obtaining a focused image on the image pick-up surface 4, the drive circuit 12 controls a motor (not shown) to rotate in a forward or reverse direction according to the amount of shift calculated so as to move the focusing lens system 1a along its optical axis.
In such an automatic focus detecting device, there is, however, a problem that since the amount of shift between two images of an object is affected remarkably by the ratio of the zooming lens system, the correlation signal weakens due to a great amount of shift when using a zooming lens of high ratio. Especially it may occur that the A data and B data are not overlapped at all when the amount of shift is extremely great, resulting in a failure of focus detection. For solving such problems without reducing resolving power in the focus detection, it may be preferable to increase pixels contributing focus detection so as to ensure a large amount of shift between the two images. However, the means of solving the problem described above still leaves some problems to be solved. One of the problems is that, in devices which are adapted to effect a focus detection on the basis of a correlation between two time sequence video signals from an image sensor, the number of bits to be compared strongly affects focus detection speed and/or electricity consumption. Another problem is the difficulty of application of the means to a video camera which require high speed focus detection for a moving object and less electricity consumption for compactness.